Reproductive characteristics of childhood cancer cases, controls, and their siblings were examined using data from a case-control study in the Denver, Colorado metropolitan area. Childhood cancer patients (n = 356) diagnosed from 1976 to 1983 were identified, and 242 were interviewed. Controls were selected by random digit dialing, with 212 interviews being completed (60% of eligibles). Extremes of birth weight were more common only among brain cancer cases. Patients were more often born preterm, particularly those with brain tumors [odds ratio (OR) = 6.1; 95% confidence interval (CI), 1.6-23.4] and were more likely to have birth defects (OR = 2.1; 95% CI, 0.9-5.0). Twins were more common among case siblings (OR = 2.6; 95% CI, 0.8-8.2). Low birth weight and preterm delivery among siblings were related only to soft tissue sarcoma. Birth defects were more common among case siblings, particularly leukemia cases (OR = 3.2; 95% CI, 1.3-7.7). Previous reports of elevated birth weight among cases and increased risk of miscarriage in case mothers were not corroborated, but associations with preterm delivery, high birth order, and birth defects among cases and birth defects and twinning among cases siblings encourage additional evaluation.